Optical Measurement Methods:
to develop novel optical measurement methods for
solving problems in critical and emerging technology
areas.
INTENDED OUTCOME AND
BACKGROUND
The Division strives to improve the accuracy, acceptability, and
utility of optical measurements by conducting long-term, directed research in
the NIST strategic focus areas of homeland security, nanotechnology, and health
care, and in such promising technology growth areas as interface science,
biophysics, semiconductor manufacturing, and quantum communication.
In the areas of nanotechnology and interface science, we are developing linear
and non-linear laser and near-field techniques to characterize thin-films,
surfaces, and interfaces important in molecular biology, polymer science, and
nanotechnology.
In biophysics and heath care, new optical methods based on surface-enhanced
Raman spectroscopy and confocal, fluorescent, imaging microscopy are being
developed for the investigation of single, biological molecules. This is part
of a larger, inter-laboratory competence program in Single-Molecule Measurement
and Manipulation, funded by the NIST Director.
The Division's activities in support of semiconductor manufacturing include
research to improve the accuracy of temperature measurements in rapid thermal
processing, to develop optical-scattering metrology for the next-generation
wafer inspection tools, and to develop chemical diagnostic methods for reactive,
ion-etching plasmas using submillimeter spectroscopy. Ultraviolet-radiation
metrology is being developed to ensure a measurement infrastructure for
157 nm and shorter-wavelength lithography, leveraging the unique
ultraviolet-radiometry capabilities of SURF III to target needs in
optical-materials properties, detector radiation-damage characterization, and
laser-power measurements.
In response to recent events, our support of optical-radiation measurements for
homeland security has increased, building on expertise developed through the
NIST Director's competence program in THz Metrology. We are investigating
femtosecond-pulsed THz spectroscopy for the detection of biological-warfare
agents in paper envelopes that are effectively transparent in the THz spectral
region, and continuous-wave THz spectroscopy for the sensitive detection of
chemical-warfare agents.
We are also using our unique expertise in correlated-photon radiometry to
develop novel, single-photon-on-demand sources for secure quantum cryptography,
in collaboration with other PL Divisions.
Accomplishments
Optical Scattering by Nanoparticles on Si Wafers
Light-scattering methods were developed to allow accurate measurement of the
diameters of standard reference particles bound to silicon substrates. This was
in response to the semiconductor industry's need for improved metrology of
particles and other defects on silicon wafers. The identification and
quantification of such defects are required to facilitate the transfer of
wafers from the factory to the chip manufacturers, and to locate and diagnose
problems in the chip fabrication line.
To calibrate inspection tools, and thus to assure agreement between the wafer
and chip manufacturers, the industry intentionally deposits accurately-sized,
polystyrene spheres onto reference wafers. Because the deposition process can
lead to changes in the size distribution of the particles, techniques are
required to accurately determine the diameters of the deposited particles.
To address this need, we did a combined theoretical and experimental
investigation of the optical properties of subwavelength-diameter spheres on
surfaces. The Bobbert-Vlieger theory of light scattering by a spherical
particle on a flat substrate was extended to account for films on both the
substrate and the particle, and then validated by measurements on deposited,
polystyrene and copper nanospheres. The copper spheres provided a particularly
demanding test of the theory due to the presence of a strong near-field
interaction between the conducting spheres and the silicon substrate.
To assess the measurement uncertainty of the diameter of the particles, the
effects of non-sphericity, size distribution, and doublet formation were
investigated. The modal diameter of the 100 nm polystyrene sphere standard
(SRM® 1963) was determined to be 99.7 nm with an
uncertainty of 1.7 nm (k = 2), in excellent agreement
with aerosol measurements. The technique is presently being incorporated into
semiconductor industry standards.
Single-Molecule Optical Probe of Binding in Antibody-Antigen Force
Measurements
As part of a NIST-wide competence program in Single-Molecule Measurement and
Manipulation (SM3), we are developing sensitive, single-molecule,
spectroscopic and imaging techniques for incorporation into (Micro Electro
Mechanical Systems) MEMS-based, molecular-screening platforms.
One technique recently developed uses single-molecule fluorescence spectroscopy
to monitor atomic-force measurements. Individual Alexa-488 dye molecules are
tethered to a glass substrate in an aqueous buffer solution. The sharpened tip
of a microfabricated cantilever, with Alexa-488 antibodies attached by
200 nm long polymer tethers, is positioned over the dye-prepared surface.
As the tip is lowered onto the surface, the cantilever deflection and the
laser-driven single-molecule fluorescence from the surface dye molecules are
monitored (Fig. 2). At some time during an approach, an Alexa antibody may
bind to an Alexa dye molecule, quenching its fluorescence. The shape of the
force curve and the recovery of fluorescence verify binding as the cantilever
is retracted and the bond is broken.
Figure 2. (A) Fluorescence image of single Alexa dye molecules on glass.
(B) Illustration of a simultaneous force and optical data sequence,
showing how cantilever deflection changes as the tip approaches, binds, and
retracts. |
Force assays are widely used in biology to elucidate binding and folding
dynamics of proteins and DNA or RNA. The addition of independent, optical
verification of single-molecule binding helps distinguish true binding events
from other interactions that commonly interfere with these assays.
In situ Nonlinear Vibrational Spectroscopy for Biological
Interfaces
In collaboration with Chemical Science and Technology Laboratory (CSTL), Doubly
Resonant Sum Frequency Generation (DR-SFG) spectroscopy is being developed as a
sensitive molecular probe of biological interfaces important for biosensors,
DNA arrays, tissue-engineering research, and the understanding of cell-membrane
structure and function.
The method relies on the enhanced nonlinear mixing of infrared and ultraviolet
or visible laser beams at an interface when both lasers are resonant with
molecular transitions, typically a vibrational transition for the infrared
laser and an electronic transition for the visible or ultraviolet laser.
Femtosecond laser technology and nonlinear optics are used to generate
spectrally broad, infrared pulses between 2.5 µm and 12 µm, which are
mixed with picosecond ultraviolet or visible laser pulses at the interface to
generate an entire broadband DR-SFG spectrum at high signal-to-noise ratio.
| |
Figure 3. DR-SFG spectra of DNA bases A, C, and G tethered to the
surface via a thiolate linkage: Au-S-(CH2)6-Base-Base. |
The approach was used to measure the first ultraviolet (270 nm) DR-SFG
spectra of DNA base dimers tethered to a gold-coated surface using thiol
attachment chemistry. Figure 3 presents DR-SFG spectra of oligomers of the
DNA bases adenine (A), cytosine (C), and guanosine (G), all of
which have electronic transitions in the 260 nm to 270 nm range to
enhance the SFG effect. The vibrational modes were identified by analogy to
ultraviolet resonance-Raman spectra at the same 270 nm wavelength. For
instance, the intense feature for adenine near 1600 cm-1
corresponds predominantly to the in-plane stretching motion of the carbon
atoms, C4 and C5, linking the double ring of the purine.
The dependence of the amplitude of DR-SFG features on the polarization direction
of the laser beams will permit the spatial orientation of the molecules to be
deduced. The method is being extended further into the ultraviolet to
200 nm to allow the investigation of peptides and proteins.
Terahertz Spectroscopy of Biological Molecules
Novel THz spectroscopy and imaging methods are being developed and demonstrated
as part of a NIST competence program on Advanced Terahertz Metrology. The
methods are being applied to characterize the molecular structures,
intramolecular force fields, low-frequency concerted motions, and
conformational dynamics of biological molecules.
Continuous-wave and pulsed THz spectra were recorded for a large number of
amino acids, peptides, sugars, carbohydrates, and vitamins at room temperature
and at liquid-helium temperature. The complex spectra reveal distinct
absorption features and patterns that provide information on the molecular
conformations and the vibrational and intramolecular force fields.
A collaborative study with the NIST Center for Neutron Research and the
Institut Laue-Langevin in Grenoble, France, is
attempting to interpret these spectra, building on a previous, successful
modeling of the neutron-scattering, vibrational spectrum of crystalline glucose.
The unique signatures of these biological molecules has led to an effort to use
THz spectroscopy for identifying chemical- and biological-warfare agents in
paper and plastic packaging that is transparent to THz radiation. We have
measured the THz spectra of about one hundred common materials and biological
samples and compiled a modest database for this purpose. This Defense Advanced
Research Project Agency (DARPA)-funded project is being undertaken in
collaboration with SPARTA, Inc.
Single-Photon-On-Demand Sources for Quantum Cryptography
The security of quantum cryptography and communication schemes depends on the
use of single photons to carry information. Parametric down-conversion (PDC),
which produces photons in correlated pairs, is the basis for one type of
single-photon source. Unfortunately, present single-photon sources are generally
incapable of producing single photons on demand with high probability, while
simultaneously suppressing the probability of yielding two or more photons.
This compromises the overall security of the communication. One reason
PDC-based schemes have this problem is because they employ photon-counting
detectors which cannot discriminate whether just one or a burst of photons was
detected.
Figure 4. Multiplexed version of a parametric down-conversion scheme for
producing single photons on demand. |
In response to the need for an improved, on-demand, single-photon source, we
have proposed a multiplexed version of the PDC scheme that allows independently
adjustable probabilities for producing one and more than one photon. The system
operates by collecting multiple pairs of correlated photons from the ring of
correlated photon pairs azimuthally distributed around the PDC pump-laser
propagation axis, as pictured in Fig. 4. The scheme allows a single,
conventional, photon-counting detector to better approximate a true
"photon-number" detector, which in turn allows the overall system to
better approximate a true single-photon source.
A recent experimental test of this concept with four channels was successful.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2002" - Table of Contents |
